The present invention relates to image formation devices printers, plotters, facsimile machines, and other imaging/copying/printing devices that form images by using a photosensitive transfer type heat developing mediums and their photosensitive medium shuttle methods.
1. Description of the Prior Art
Color image formation device, such as is disclosed in Japanese Laid-Open Patent Publication 62-147461, have a photosensitive medium supply section, an exposure section, a heat developing section, a pressure transfer section and a photosensitive medium disposal section are provided along the medium pathway. Photosensitive medium is stored rolled up on a drum (roll), and sheets are cut an appropriate length for immediate use in an exposure assembly. There, light reflected from an original document is focused to form a latent image on the medium. Heat, at 80.degree. to 200.degree. C. applied for about 30 seconds, is then used to develop the image. A pressure transfer section presses together the photosensitive medium with a transfer medium (e.g., paper). The developed image transfers because of the pressure. The two mediums are then separated, the printed image is output to a stacking bin and the used photosensitive medium is wound up on a spool.
Photosensitive mediums typically use plastic films for their base layer. The surface is coated with a variety of microcapsules filled with a photosensitive solution and various color pigments. A photo-reaction occurs during exposure and heat in a developer causes certain color pigments to become selectively hardened, depending on the image and its colors. During pressure transfer, both photo-reacted and non-photoreacted microcapsules are crushed. The pigments that were not selectively hardened by the exposure and development will be transferred, thus forming an output image.
Prior art devices that cut the photosensitive medium first and then use it have very complex paper and film handling mechanisms, and copy output delays. It would be better to have the photosensitive medium spooled between two rolls such that it can be used in a continuous belt, without the need to cut the medium into sheets, developing time would be shortened. But it is not that simple, the problems spelled out below can be expected in a continuous belt system.
When a photosensitive medium is heated to the high temperatures needed for developing, wrinkling in the medium is a major problem. The high heat causes thermal stretch and shrink. The longitudinal dimension changes can be absorbed by the spooling, but the changes in the width are harder to deal with. So lengthwise wrinkles are almost unavoidable. Three distinct temperature states in the medium occur while it is in the vicinity of the heat developer. Residual distortions from the process of making the medium can remain in the medium, so wrinkles may not disappear even after the medium has been sufficiently cooled. In other words, the pressure transfer stage may not be able to avoid wrinkles, which can degrade the quality of the output image. The effect of very small wrinkles can be compensated for by adjusting the developing time or transfer pressure. Small wrinkles can cause photosensitive medium to lift, which causes uneven exposures.
Experiments have been run regarding wrinkles in mediums based on polyethylene terephthalate (PET) film. In one test, tension that was applied to the medium was increased in steps. Observations were taken for the number of wrinkles and the amount of thermal shrinkage. A photosensitive medium 215 mm wide was used. The heat developing temperature was a constant 150.degree. C., and the shuttle speed of photosensitive medium was 10 mm/s. Normally, transfer pressure is applied without the aid of heat. But in a second experiment, wrinkles for various heat developing temperatures and pressure transfer temperatures were tried while the shuttle speed was varied to inject a variety of delays between development and image transfer points in time. The results of the first experiment are shown in Table I and those of the second experiment in Table II.
TABLE I ______________________________________ Uneven exposure due to Thermal shrinkage of wrinkles photosensitive medium caused by Film Lengthwise Widthwise thermal tension direction direction shrinkage ______________________________________ 0 gf 1.2% 0.7% bad 200 1.1 0.6 bad 400 0.8 0.5 acceptable 600 0.5 0.5 Good 800 0.5 0.5 Good ______________________________________
TABLE II ______________________________________ Heat Pressure Wrinkling versus time delay between developing transfer developing and transfer temp. temp. 1 sec 2 sec 5 sec 10 sec 20 sec ______________________________________ 155.degree. C. 30.degree. C. none none none none small 155.degree. C. 50.degree. C. none none none none wrinkled 155.degree. C. 70.degree. C. none none none small wrinkled 155.degree. C. 90.degree. C. none none none small wrinkled 155.degree. C. 110.degree. C. none none small wrinkled wrinkled 135.degree. C. 50.degree. C. none none none none wrinkled 135.degree. C. 70.degree. C. none none none small wrinkled 135.degree. C. 90.degree. C. none none small wrinkled wrinkled ______________________________________
According to Table I, wrinkles (and the problems caused by them) can be eliminated by applying a constant tension to the medium. The amount of thermal shrinkage can be limited to less than one percent. Preferably, tension should be applied both the length and width, to balance the changes in two dimensions. According to Table II, wrinkles can be minimized by pressure transferring much cooling has occurred after heat developing. The temperature difference between a heat developing temperature and a pressure transfer temperature, divided by the shuttle time in seconds from the heat developer to the point of transfer pressure, should be greater than 10.degree. C. The reason for this is the photosensitive medium will stretch again if it has to be re-heated, such as can occur when it has been allowed to cool after developing.